Currently, honeycomb seals are used in a number of different locations in gas turbines, for example in stages 2 and 3 of the 7E turbine against rails on shrouded buckets. Honeycomb is used as an abradable system, whereby the rotating rails on the buckets may incur into them during any transient closures between the buckets and the shroud. The incursion into the honeycomb is a sacrificial form of sealing, as the intention is that there be no damage to the bucket rails during the interaction. This allows for tightest possible clearances during the steady state operation of the gas turbine. However, the primary problem with honeycomb, particularly in stage 2 shrouds, is its low resistance to oxidation. The honeycomb material is typically 75Ni-16Cr-4.5Al-3Fe-0.05C-0.01Y-0.5Mn-0.2Si-0.1Zr-0.01B (Haynes 214). The temperature range in this location of the turbine is generally in the range of 1500-1700° F. At ˜1600° F., the oxidation life of a 0.005″ thick honeycomb is less than 20,000 hours.
An alternative Sealing system currently in use in turbines, particularly gas turbines, consist of a known material called “fiber metal,” which is typically Hastelloy-x or a FeCrAlY compound. Because fiber metal is directly exposed to the high temperature gases of the second and third stages, its oxidation life is limited.
A need exists for an improved method of assembly of honeycomb and fiber metal when coated with Al2O3 to substrates, such as Stage 2 and Stage 3 shrouds. Application of Al2O3 on honeycomb and fiber metal reduces the wetting of the seal surface by braze material, which leads to poor bonding between the seal and the substrate. The present invention seeks to fill that need.